Unable to start a QProcess again

[Pothos]

I am an experimenting with different APIs of QT. In the private-section of my class I have a new process instance declared as follows:

QProcess *myProcess;

Then I initialize it in the constructor of said class as follows:

myProcess = new QProcess();

I have a button in the GUI that executes the program rsync when pressed. Here is the most relevant section of the code that it executes:

if (myProcess->state() != QProcess::NotRunning) {
      qDebug() << "Can't start a new process while the old one is still running.";
      return;
}

connect(myProcess, &QProcess::readyReadStandardOutput, [=](){
        ui->consoleTextBrowser->append(myProcess->readAllStandardOutput());
});

connect(myProcess, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), [=](int exitCode, QProcess::ExitStatus exitStatus) {
        qDebug() << exitCode;
        qDebug() << exitStatus;
        myProcess->deleteLater();
});

myProcess->start(programString, rsyncArguments);

The purpose of the if-sentence in the beginning is to make sure that I am unable to start a new rsync process. Or at least that is my intention.

Everything works fine the first time I run this. However, when I run this for the second time, the program crashes due to a segmentation fault. To my understanding the crash happens when the first connect is called.

What am I doing wrong here? Am I using these connect-calls the wrong way? Should I somehow disconnect them before running rsync again?

[SGaist]

Hi,

@Pothos said in Unable to start a QProcess again:

connect(myProcess, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), [=](int exitCode, QProcess::ExitStatus exitStatus) {
qDebug() << exitCode;
qDebug() << exitStatus;
myProcess->deleteLater();
});

You're nuking your myProcess object and likely not recreating it.

[Pothos]

@SGaist
Oh, well damn it. I thought I was supposed to do that. Seems to fix the issue...

However, I have an additional question related to this exact same topic:
How should I terminate an ongoing rsync-process started by this method, if I want to be able to run it again if I so choose? Is myProcess->terminate() safe to call mid-execution?

[SGaist]

Terminate sends a SIGKILL which is likely not what you want.

My memory failed me (good to re-read the documentation from time to time). So SIGTERM is used by QProcess.

You can use the standard kill method with SIGINT on the pid of the application that you can get from QProcess.

[Pothos]

@SGaist
Thanks. Does SIGKILLed rsync leave any unnecessary files left or cause memory leak though?

[SGaist]

Memory leak should not happen however there will be no cleanup done. SIGKILL does not ask your process to stop in a nice and polite manner (that's the role of SIGINT), it executes it.

[JonB]

@SGaist
What do you think about sending SIGTERM rather than SIGINT?

https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html

Macro: int SIGTERM

The SIGTERM signal is a generic signal used to cause program termination. Unlike SIGKILL, this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate.

The shell command kill generates SIGTERM by default.

Macro: int SIGINT

The SIGINT (“program interrupt”) signal is sent when the user types the INTR character (normally C-c). See Special Characters, for information about terminal driver support for C-c.

https://www.quora.com/What-is-the-difference-between-the-SIGINT-and-SIGTERM-signals-in-Linux-What’s-the-difference-between-the-SIGKILL-and-SIGSTOP-signals

SITERM
According to ISO/IEC 9899:2011: a termination request sent to the program *

SIGINT
According to ISO/IEC 9899:2011: receipt of an interactive attention signal *

@Pothos
You can hope/assume rsync will clean itself up. So long as you don't actually nuke it!

You should test which signals work best with it.

[SGaist]

@JonB excellent point, that's indeed also a good if not better idea.

[Pothos]

@SGaist
@JonB
I see. So QProcess::terminate() is the way to go since it sends a SIGTERM to the process?

[SGaist]

(Updated my answer above)

I would test and compare the two to see which one does what you want best.

[JonB]

@Pothos
Indeed:

On Unix and macOS the SIGTERM signal is sent.

[JonB]

@Pothos
You should indeed test to see rsync's behaviour. QProcess::terminate() is easy to call, I think to send a SIGINT you will have to use the Linux kill(2) function, I think @SGaist was indicating that.